The present invention relates to a method for producing switching elements used in an active matrix liquid crystal display apparatus and to a liquid crystal display apparatus produced by the method.
A liquid crystal display apparatus which is one of a flat panel display instead of CRT has been studied enthusiastically, and it has been practically used as a display apparatus for a microtelevision driven by battery and for a notebook-type personal computer by utilizing a characteristic that its power consumption is small and such an apparatus is thin.
As a method of driving a liquid crystal display apparatus, an active matrix TFT array, in which a TFT (thin film transistor) is used as a switching element, is mainly used from the aspect of high display quality.
It is effect for lowering power consumption of a liquid crystal display apparatus to increase an effective display area of a pixel portion in a liquid crystal panel, namely, to heighten an aperture ratio of the pixels.
As an effective TFT array for obtaining a liquid crystal panel with high aperture ratio, Japanese Examined Patent Publication No. 2521752, Japanese Examined Patent No. 2598420 and Japanese Unexamined Patent Publication No. 163528/1992, for example, disclose an arrangement that after a TFT composed of a scanning electrode, signal electrode and semiconductor layer is formed, an interlayer insulating film composed of transparent insulating synthetic resin is provided over the TFT, and an pixel electrode is formed as the topmost layer.
The high aperture ratio is obtained by the aforementioned arrangement due to following two points. Namely, as explained in Japanese Examined Patent Publication No. 2521752, since a pixel electrode is formed on an interlayer insulating film whose surface is made to be flat by a transparent insulating synthetic resin film, improper display (domain phenomenon) due to disordered alignment of liquid crystal molecules occurring in a level difference portion of the pixel electrode in the conventional arrangement can be eliminated, so an effective display area can be increased. Further, as explained in Japanese Patent Publication No. 2598420 and Japanese Unexamined Patent Publication No. 163528/1992, by forming the pixel electrode on a comparatively thick interlayer insulating film with thickness of 0.3 .mu.m to 2 .mu.m, electric short circuit does not occur between gate wiring/source wiring under the interlayer insulating film and the pixel electrode, so the electrode pixel can be formed with a wide area so as to be overlapped on the wirings.
The process for realizing the TFT array arrangement with high aperture ratio such as a method of forming the interlayer insulating film made of transparent insulating synthetic resin is described in detail in Japanese Unexamined Patent Publication No. 96837/1997, Japanese Unexamined Patent Publication No. 127553/1997 and Japanese Unexamined Patent Publication No. 152625/1997.
The producing steps of the TFT array with high aperture ratio are described briefly. TFT composed of a gate electrode, gate insulating film and semiconductor layer, a source electrode and a drain electrode are successively formed on a transparent insulating substrate like glass. Next, an interlayer insulating film made of transparent insulating synthetic resin is formed, and a contact hole (or contact via) is formed. Finally, a pixel electrode is formed, and thus the TFT array is finished. The pixel electrode is electrically connected with the electrode thereunder through the contact hole formed in the interlayer insulating film.
As described in Japanese Examined Patent Publication No. 127553/1997 and Japanese Examined Patent Publication No. 152625/1997, for example, there exist two types of the methods of forming the contact hole in the interlayer insulating film: the method using photosensitive transparent resin and the method using non-photosensitive transparent resin.
In the case of using photosensitive transparent resin, a desired contact hole is formed by the same step as photolithography in which after resin is applied and baked, it is exposed and developed by using a mask pattern of a contact hole. Meanwhile, in the case of using non-photosensitive transparent resin, after resin is applied and baked, a resist is applied and a contact pattern is formed by photolithography, and dry etching is performed by gas including at least one of CF.sub.4, CF.sub.3 and SF.sub.6 so that a desired contact hole is obtained by removing the resist.
Needless to say, in the case of using photosensitive resin, a contact hole may be formed by dry etching using a resist mask. Moreover, a material of the transparent insulating synthetic resin film may be an organic material or inorganic material.
It is necessary for display quality in the above TFT array with high aperture ratio that the electrical connection between the pixel electrode on the upper layer and the drain electrode on the lower layer through the contact hole in the interlayer insulating film is satisfactory. The satisfactory electric connection is represented generally by contact resistivity of the pixel electrode and drain electrode on the surface of the contact hole, and the contact resistivity per aperture area of 50 .mu.m.quadrature. should be not more than 10E4.OMEGA. (hereinafter, the value of the contact resistivity is a value per aperture ratio of 50 .mu.m.quadrature. as long as no explanatory note).
However, in the conventional process for the TFT array with high aperture ratio using the interlayer insulating film made of transparent resin, it is difficult to stably obtain the value of the contact resistivity of not more than 10E4.OMEGA., so there arises a problem that yield is lowered.
In particular, since the contact hole for connecting the pixel electrode with the drain electrode is formed in a whole pixel portion where the pixels are arranged in matrix pattern on the substrate, even if improper contact resistivity occurs partially on the substrate, improper display occurs, and thus the yield is lowered. Therefore, it is very important that the step for forming the contact hole which stably provides low contact resistivity is realized.
However, the method of forming the contact via in the interlayer insulating film which provides stably low contact resistivity is not concretely disclosed in the aforementioned patent publications. Moreover, such a method is not described also in another relating patent documents.
It is an object of the present invention to provide a method of producing a TFT array which is capable of solving the above problem and of stably maintaining the value of contact resistivity of a pixel electrode and drain electrode through a contact hole in an interlayer insulating film at not more than 10E4.OMEGA. in a TFT array with high aperture ratio using the interlayer insulating film made of transparent resin, and relates to a liquid crystal display apparatus produced by this method.